1. Field of the Invention
The present invention relates to a coordinates input apparatus and, more particularly, to a coordinates input apparatus in which the vibration which is input by a vibration pen is detected by a plurality of sensors attached to a vibration propagating plate and the coordinates of the vibration pen on the vibration propagating plate are detected.
2. Related Background Art
Various kinds of input apparatuses have conventionally been known as coordinates input apparatuses. In this kind of apparatus, a coordinate system is set on a predetermined input surface, coordinates are input onto the input surface by an input device of a predetermined system, and the coordinate information on the coordinate system on the input surface is detected.
As a detecting system, there is known a system in which an input tablet is constituted by arranging a resistive film and a conductive film so as to face each other, and these films are touched by a writing tool such as finger, pen, or the like, or a system in which an input member consisting of an ultrasonic pen or the like is used, an ultrasonic vibration is input to an input tablet consisting of a vibration propagating plate or the like, and the coordinate values are detected from the vibration propagating time of the elastic wave which is generated on the tablet, or the like.
According to the latter system by the vibration propagation, a plurality of vibration sensors consisting of a plurality of piezoelectric elements or the like each for converting the mechanical vibration to the electric signal are attached to the glass plate constituting the tablet and the other vibration propagating plate in order to calculate the propagating time of the vibration which is propagated.
If the vibration input timing has already been known, by detecting the arrival timing of the vibration to the vibration sensor, the vibration propagating time to the sensor can be known. Since the vibration propagating speed on the vibration propagating plate is considered to be constant, the distance of the straight line between the vibration sensor and the input point can be obtained from the vibration propagating time. If the distance of the straight line between each sensor and the input point is known, the coordinate values of the input point can be determined by the theorem of three squares or the like.
According to such conventional systems, a method whereby the peak value of the detection signal waveform which is output from the vibration sensor is detected is used to decide the vibration detecting timing.
However, the vibration propagating plate has limited area and some reflected waves are certainly generated in the edge portion areas. Therefore, the vibration waveform which is input to the vibration sensor is the synthesis wave of the direct wave and the reflected waves.
In particular, the difference between the paths of the direct wave and reflected wave is very small in dependence on the positional relations among the coordinate input point, the sensor, and the edge portions of the vibration propagating plate. The detection signal waveform is largely distorted due to the interference between them. There is a problem such that a deviation occurs in the detection timing due to this influence and the coordinate detecting accuracy deteriorates.
There is also known a technique to support the edge portions of the vibration propagating plate by a vibration proofing material or the like in consideration of the foregoing point. However, it is difficult to perfectly eliminate the reflected waves. The area of the effective input surface is made smaller than the size of the vibration propagating plate, thereby allowing the direct wave and the reflected waves to reach the sensor with time lags, and the occurrence of the deviation of the detection timing due to the interference must be prevented.
Therefore, to assure the area of the necessary effective input surface, the area of the vibration propagating plate must be set to be a large value. There is a problem such that the size of the apparatus increases or the area of the effective input surface is limited.